Rate Responsive Leadless Cardiac Pacemaker

ABSTRACT

A leadless cardiac pacemaker comprises a housing, a plurality of electrodes coupled to an outer surface of the housing, and a pulse delivery system hermetically contained within the housing and electrically coupled to the electrode plurality, the pulse delivery system configured for sourcing energy internal to the housing, generating and delivering electrical pulses to the electrode plurality. The pacemaker further comprises an activity sensor hermetically contained within the housing and adapted to sense activity and a processor hermetically contained within the housing and communicatively coupled to the pulse delivery system, the activity sensor, and the electrode plurality, the processor configured to control electrical pulse delivery at least partly based on the sensed activity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/549,603, filed Oct. 13, 2006, now U.S. Pat. No. 7,937,148; whichapplication claims the benefit of priority to and incorporates herein byreference in its entirety for all purposes, U.S. Provisional ApplicationNos. 60/726,706, entitled “LEADLESS CARDIAC PACEMAKER WITH CONDUCTEDCOMMUNICATION,” filed Oct. 14, 2005; 60/761,531, entitled “LEADLESSCARDIAC PACEMAKER DELIVERY SYSTEM,” filed Jan. 24, 2006; 60/729,671,entitled “LEADLESS CARDIAC PACEMAKER TRIGGERED BY CONDUCTEDCOMMUNICATION,” filed Oct. 24, 2005; 60/737,296, entitled “SYSTEM OFLEADLESS CARDIAC PACEMAKERS WITH CONDUCTED COMMUNICATION,” filed Nov.16, 2005; 60/739,901, entitled “LEADLESS CARDIAC PACEMAKERS WITHCONDUCTED COMMUNICATION FOR USE WITH AN IMPLANTABLECARDIOVERTER-DEFIBRILLATOR,” filed Nov. 26, 2005; 60/749,017, entitled“LEADLESS CARDIAC PACEMAKER WITH CONDUCTED COMMUNICATION AND RATERESPONSIVE PACING,” filed Dec. 10, 2005; and 60/761,740, entitled“PROGRAMMER FOR A SYSTEM OF LEADLESS CARDIAC PACEMAKERS WITH CONDUCTEDCOMMUNICATION,” filed Jan. 24, 2006; all by Peter M. Jacobson.

BACKGROUND

Cardiac pacing electrically stimulates the heart when the heart'snatural pacemaker and/or conduction system fails to provide synchronizedatrial and ventricular contractions at appropriate rates and intervalsfor a patient's needs. Such bradycardia pacing provides relief fromsymptoms and even life support for hundreds of thousands of patients.Cardiac pacing may also give electrical overdrive stimulation intendedto suppress or convert tachyarrhythmias, again supplying relief fromsymptoms and preventing or terminating arrhythmias that could lead tosudden cardiac death.

Cardiac pacing is usually performed by a pulse generator implantedsubcutaneously or sub-muscularly in or near a patient's pectoral region.The generator usually connects to the proximal end of one or moreimplanted leads, the distal end of which contains one or more electrodesfor positioning adjacent to the inside or outside wall of a cardiacchamber. The leads have an insulated electrical conductor or conductorsfor connecting the pulse generator to electrodes in the heart. Suchelectrode leads typically have lengths of 50 to 70 centimeters.

Known pulse generators can include various sensors for estimatingmetabolic demand, to enable an increase in pacing rate proportional andappropriate for the level of exercise. The function is usually known asrate-responsive pacing. For example, an accelerometer can measure bodymotion and indicate activity level. A pressure transducer in the heartcan sense the timing between opening and closing of various cardiacvalves, or can give a measure of intracardiac pressure directly, both ofwhich change with changing stroke volume. Stroke volume increases withincreased activity level. A temperature sensor can detect changes in apatient's blood temperature, which varies based on activity level. Thepacemaker can increase rate proportional to a detected increase inactivity.

Pulse generator parameters are usually interrogated and modified by aprogramming device outside the body, via a loosely-coupled transformerwith one inductance within the body and another outside, or viaelectromagnetic radiation with one antenna within the body and anotheroutside.

Although more than one hundred thousand rate-responsive pacemakers areimplanted annually, various well-known difficulties are present.

The pulse generator, when located subcutaneously, presents a bulge inthe skin that patients can find unsightly or unpleasant. Patients canmanipulate or “twiddle” the device. Even without persistent twiddling,subcutaneous pulse generators can exhibit erosion, extrusion, infection,and disconnection, insulation damage, or conductor breakage at the wireleads. Although sub-muscular or abdominal placement can address some ofconcerns, such placement involves a more difficult surgical procedurefor implantation and adjustment, which can prolong patient recovery.

A conventional pulse generator, whether pectoral or abdominal, has aninterface for connection to and disconnection from the electrode leadsthat carry signals to and from the heart. Usually at least one maleconnector molding has at least one terminal pin at the proximal end ofthe electrode lead. The at least one male connector mates with at leastone corresponding female connector molding and terminal block within theconnector molding at the pulse generator. Usually a setscrew is threadedin at least one terminal block per electrode lead to secure theconnection electrically and mechanically. One or more O-rings usuallyare also supplied to help maintain electrical isolation between theconnector moldings. A setscrew cap or slotted cover is typicallyincluded to provide electrical insulation of the setscrew. The complexconnection between connectors and leads provides multiple opportunitiesfor malfunction.

For example, failure to introduce the lead pin completely into theterminal block can prevent proper connection between the generator andelectrode.

Failure to insert a screwdriver correctly through the setscrew slot,causing damage to the slot and subsequent insulation failure.

Failure to engage the screwdriver correctly in the setscrew can causedamage to the setscrew and preventing proper connection.

Failure to tighten the setscrew adequately also can prevent properconnection between the generator and electrode, however over-tighteningof the setscrew can cause damage to the setscrew, terminal block, orlead pin, and prevent disconnection if necessary for maintenance.

Fluid leakage between the lead and generator connector moldings, or atthe setscrew cover, can prevent proper electrical isolation.

Insulation or conductor breakage at a mechanical stress concentrationpoint where the lead leaves the generator can also cause failure.

Inadvertent mechanical damage to the attachment of the connector moldingto the generator can result in leakage or even detachment of themolding.

Inadvertent mechanical damage to the attachment of the connector moldingto the lead body, or of the terminal pin to the lead conductor, canresult in leakage, an open-circuit condition, or even detachment of theterminal pin and/or molding.

The lead body can be cut inadvertently during surgery by a tool, or cutafter surgery by repeated stress on a ligature used to hold the leadbody in position. Repeated movement for hundreds of millions of cardiaccycles can cause lead conductor breakage or insulation damage anywherealong the lead body.

Although leads are available commercially in various lengths, in someconditions excess lead length in a patient exists and is to be managed.Usually the excess lead is coiled near the pulse generator. Repeatedabrasion between the lead body and the generator due to lead coiling canresult in insulation damage to the lead.

Friction of the lead against the clavicle and the first rib, known assubclavian crush, can result in damage to the lead.

In many applications, such as dual-chamber pacing, multiple leads can beimplanted in the same patient and sometimes in the same vessel. Abrasionbetween the leads for hundreds of millions of cardiac cycles can causeinsulation breakdown or even conductor failure.

Communication between the implanted pulse generator and externalprogrammer uses a telemetry coil or antenna and associated circuitry inthe pulse generator, adding complexity that increases the size and costof devices. Moreover, power consumption from the pulse generator batteryfor communication typically exceeds power for pacing by one or moreorders of magnitude, introducing a requirement for battery powercapability that can prevent selecting the most optimal batteryconstruction for the otherwise low-power requirements of pacing.

SUMMARY

According to an embodiment of a biostimulation system, a leadlesscardiac pacemaker comprises a housing, a plurality of electrodes coupledto an outer surface of the housing, and a pulse delivery systemhermetically contained within the housing and electrically coupled tothe electrode plurality, the pulse delivery system configured forsourcing energy internal to the housing, generating and deliveringelectrical pulses to the electrode plurality. The pacemaker furthercomprises an activity sensor hermetically contained within the housingand adapted to sense activity and a processor hermetically containedwithin the housing and communicatively coupled to the pulse deliverysystem, the activity sensor, and the electrode plurality, the processorconfigured to control electrical pulse delivery at least partly based onthe sensed activity.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention relating to both structure and method ofoperation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1A is a pictorial diagram showing an embodiment of a cardiac pacingsystem that includes a rate-responsive leadless cardiac pacemaker;

FIG. 1B is a schematic block diagram showing interconnection ofoperating elements of an embodiment of the illustrative rate-responsiveleadless cardiac pacemaker;

FIG. 2 is a pictorial diagram showing the physical location of someelements of an embodiment of a rate-responsive leadless cardiacpacemaker;

FIG. 3 is a pictorial diagram that depicts the physical location of someelements in an alternative embodiment of a rate-responsive leadlesscardiac pacemaker;

FIG. 4 is a time waveform graph illustrating a conventional pacingpulse;

FIG. 5 is a time waveform graph depicting a pacing pulse adapted forcommunication as implemented for an embodiment of the illustrativepacing system;

FIG. 6 is a time waveform graph showing a sample pulse waveform usingoff-time variation for communication;

FIG. 7 is a schematic flow chart depicting an embodiment of a method foroperating an activity sensor in a rate-responsive cardiac pacemaker; and

FIG. 8 is a schematic flow chart showing an embodiment of a method forcommunicating information for setting control parameters for an activitysensor in a cardiac pacing system.

DETAILED DESCRIPTION

In an illustrative system, a rate-responsive leadless cardiac pacemakercan be implanted adjacent to the inside or outside wall of a cardiacchamber.

In addition, a technique for rate-responsive pacing enables pacingcontrol of the leadless cardiac pacemaker which is implanted adjacent tothe inside or outside wall of a cardiac chamber.

A cardiac pacemaker for implantation in the human body, morespecifically a leadless cardiac pacemaker for implantation adjacent tothe inside or outside wall of a cardiac chamber, uses two or moreelectrodes located within, on, or within two centimeters of the housingof the pacemaker for pacing and sensing at the cardiac chamber and forbidirectional communication with at least one other device within oroutside the body. The pacemaker contains an activity sensor, such as anaccelerometer, temperature sensor, and/or a pressure transducer tomeasure patient activity, enabling rate-responsive pacing.

The illustrative system enables cardiac pacing without a pulse generatorlocated in the pectoral region or abdomen, without an electrode-leadseparate from the pulse generator, without a communication coil orantenna, and without an additional requirement on battery power fortransmitted communication.

An illustrative rate-responsive leadless cardiac pacemaker can besubstantially enclosed in a hermetic housing suitable for placement onor attachment to the inside or outside of a cardiac chamber. Thepacemaker has at least two electrodes located within, on, or near thehousing, for delivering pacing pulses to and sensing electrical activityfrom the muscle of the cardiac chamber, and for bidirectionalcommunication with at least one other device within or outside the body.The housing contains a primary battery to provide power for pacing,sensing, and communication. The housing also contains circuits forsensing cardiac activity from the electrodes, receiving information fromat least one other device via the electrodes, generating pacing pulsesfor delivery via the electrodes, transmitting information to at leastone other device via the electrodes, monitoring device health, andcontrolling these operations in a predetermined manner.

A leadless pacemaker is configured for implantation adjacent to theinside or outside wall of a cardiac chamber, without the need for aconnection between the pulse generator and electrode lead, and withoutthe need for a lead body.

In some embodiments, the illustrative system enables communicationbetween the implanted pulse generator and a device internal or externalto the body, using conducted communication via the same electrodes usedfor pacing, without the need for an antenna or telemetry coil.

Still other embodiments enable communication between the implanted pulsegenerator and a device internal or external to the body, with powerconsumption similar to that for cardiac pacing, to allow optimization ofbattery performance.

Referring to FIGS. 1A and 1B, a pictorial view which is not shown toscale and a schematic block diagram respectively depict an embodiment ofa cardiac pacing system 100 that comprises a rate-responsive leadlesscardiac pacemaker 102. The rate-responsive leadless cardiac pacemaker102 comprises a housing 110, multiple electrodes 108 coupled to thehousing 110, a pulse delivery system 152 hermetically contained withinthe housing 110 and electrically coupled to the electrodes 108. Thepulse delivery system 152 configured for sourcing energy internal to thehousing 110, generating and delivering electrical pulses to theelectrodes 108. The rate-responsive leadless cardiac pacemaker 102further comprises an activity sensor 154 which is hermetically containedwithin the housing 110 and adapted to sense activity. A processor 112 isalso hermetically contained within the housing 110 as part of a pulsedelivery system 152 and is communicatively coupled to the activitysensor 154, and the electrodes 108. The processor 112 can controlelectrical pulse delivery at least partly based on the sensed activity.

In various embodiments, the electrodes 108 can be coupled on, within, orwithin two centimeters of the housing 110. In some arrangements, theelectrodes 108 can be formed integrally to an outer surface of thehousing 110.

Referring to FIG. 1B, the rate-responsive leadless cardiac pacemaker 102has functional elements substantially enclosed in a hermetic housing110. The pacemaker has at least two electrodes 108 located within, on,or near the housing 110, for delivering pacing pulses to and sensingelectrical activity from the muscle of the cardiac chamber, and forbidirectional communication with at least one other device within oroutside the body. Hermetic feedthroughs 130, 131 conduct electrodesignals through the housing 110. The housing 110 contains a primarybattery 114 to provide power for pacing, sensing, and communication. Thehousing 110 contains circuits 132 for sensing cardiac activity from theelectrodes 108; circuits 134 for receiving information from at least oneother device via the electrodes 108; and a pulse generator 116 forgenerating pacing pulses for delivery via the electrodes 108 and alsofor transmitting information to at least one other device via theelectrodes 108. The pacemaker 102 further contains circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138. The pacemaker 102 further containsprocessor or controller circuits 112 for controlling these operations ina predetermined manner.

In accordance with another embodiment of a pacing system, a leadlesscardiac pacemaker 102 comprises a housing 110, multiple electrodes 108coupled to the housing 108, and a pulse generator 116 hermeticallycontained within the housing 110 and electrically coupled to theelectrodes 108. The pulse generator 116 is configured to generate anddeliver electrical pulses to the electrodes 108 powered from a source114 contained entirely within the housing 110. An activity sensor 154 ishermetically contained within the housing 110 and adapted to senseactivity. A logic 112, for example a processor, controller, centralprocessing unit, state machine, programmable logic array, and the like,which is hermetically contained within the housing 110 andcommunicatively coupled to the pulse generator 116, the activity sensor154, and the electrodes 108. The logic 112 is configured to controlelectrical pulse delivery at least partly based on the sensed activity.

In some embodiments, the logic 112 can be a processor that controlselectrical pulse delivery and application of the activity sensoraccording to one or more programmable parameters with the processorprogrammable by communication signals transmitted via the electrodes108.

The information communicated on the incoming communication channel caninclude, but is not limited to pacing rate, pulse duration, sensingthreshold, and other parameters commonly programmed externally intypical pacemakers. The information communicated on the outgoingcommunication channel can include, but is not limited to programmableparameter settings, event counts (pacing and sensing), battery voltage,battery current, and other information commonly displayed by externalprogrammers used with common pacemakers. The outgoing communicationchannel can also echo information from the incoming channel, to confirmcorrect programming.

Also shown in FIG. 1B, the primary battery 114 has positive terminal 140and negative terminal 142. A suitable primary battery has an energydensity of at least 3 W·h/cc, a power output of 70 microwatts, a volumeless than 1 cubic centimeter, and a lifetime greater than 5 years.

One suitable primary battery uses beta-voltaic technology, licensed toBetaBatt Inc. of Houston, Tex., USA, and developed under a trade nameDEC™ Cell, in which a silicon wafer captures electrons emitted by aradioactive gas such as tritium. The wafer is etched in athree-dimensional surface to capture more electrons. The battery issealed in a hermetic package which entirely contains the low-energyparticles emitted by tritium, rendering the battery safe for long-termhuman implant from a radiological-health standpoint. Tritium has ahalf-life of 12.3 years so that the technology is more than adequate tomeet a design goal of a lifetime exceeding 5 years.

In accordance with another embodiment of a pacing system, a leadlesscardiac pacemaker 102 comprises a housing 110, multiple electrodes 108coupled to the housing 110, and a pulse generator 116 hermeticallycontained within the housing 110 and electrically coupled to theelectrodes 108. The pulse generator 116 generates and deliverselectrical pulses to the electrodes 108, causing cardiac contractions.The pulse generator 116 also conveys information to one or more devices106 external to the pacemaker 102. The pacemaker 102 further comprisesat least one amplifier 132, 134 hermetically contained within thehousing 110 and electrically coupled to the electrodes 108. Theamplifier or amplifiers 132, 134 are configured to amplify signalsreceived from the electrodes 108 and to detect cardiac contractions, andfurther can receive information from the external device or devices 106.The pacemaker 102 further comprises a power supply 114 hermeticallycontained within the housing 110 and coupled to the pulse generator 116.The power supply 114 sources energy for the electrical pulses frominternal to the housing 110. The pacemaker 102 has an activity sensor154 hermetically contained within the housing 110 that senses activity.A processor 112 is hermetically contained within the housing 110 andcommunicatively coupled to the pulse generator 116, the amplifiers 132,134, the activity sensor 154, and the electrodes 108. The processor 112configured to receive amplifier output signals from the amplifier oramplifiers 132, 134 and control electrical pulse delivery at leastpartly based on the sensed activity.

In an illustrative embodiment, the amplifiers comprise a cardiac sensingamplifier 132 that consumes no more than 5 microwatts, a communicationsamplifier 134 that consumes no more than 25 microwatts, and arate-response sensor amplifier 156 that consumes no more than 10microwatts.

In an example embodiment, the regulator 146 can be configured to consumeelectrical power of no more than 2 microwatts and configured to supplyelectrical power of no more than 74 microwatts in the illustrativesystem that includes a rate-response amplifier.

The processor 112 can be configured to consume electrical power of nomore than 5 microwatts averaged over one cardiac cycle.

Current from the positive terminal 140 of primary battery 114 flowsthrough a shunt 144 to a regulator circuit 146 to create a positivevoltage supply 148 suitable for powering the remaining circuitry of thepacemaker 102. The shunt 144 enables the battery current monitor 136 toprovide the processor 112 with an indication of battery current drainand indirectly of device health.

The illustrative power supply can be a primary battery 114 such as abeta-voltaic converter that obtains electrical energy fromradioactivity. In some embodiments, the power supply can be selected asa primary battery 114 that has a volume less than approximately 1 cubiccentimeter.

In an illustrative embodiment, the primary battery 114 can be selectedto source no more than 75-80 microwatts instantaneously since a higherconsumption may cause the voltage across the battery terminals tocollapse. Accordingly in one illustrative embodiment the circuitsdepicted in FIG. 1B can be designed to consume no more than a total of74 microwatts. The design avoids usage of a large filtering capacitorfor the power supply or other accumulators such as a supercapacitor orrechargeable secondary cell to supply peak power exceeding the maximuminstantaneous power capability of the battery, components that would addvolume and cost.

In various embodiments, the system can manage power consumption to drawlimited power from the battery, thereby reducing device volume. Eachcircuit in the system can be designed to avoid large peak currents. Forexample, cardiac pacing can be achieved by discharging a tank capacitor(not shown) across the pacing electrodes. Recharging of the tankcapacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

Implantable systems that communicate via long distance radio-frequency(RF) schemes, for example Medical Implant Communication Service (MICS)transceivers, which exhibit a peak power requirement on the order of 10milliwatts, and other RF or inductive telemetry schemes are unable tooperate without use of an additional accumulator. Moreover, even withthe added accumulator, sustained operation would ultimately cause thevoltage across the battery to collapse.

In various embodiments, the activity sensor 154 is adapted forcontrolling rate-responsive pacing and may use any appropriatetechnology, for the example the activity sensor 154 may be anaccelerometer, a temperature sensor, a pressure sensor, or any othersuitable sensor.

In an illustrative embodiment, the activity sensor 154 can operate witha power requirement of no more than 10 microwatts.

FIG. 1B shows a pacemaker embodiment wherein the activity sensorcomprises an accelerometer 154 and an accelerometer amplifier 156configured to detect patient activity for rate-responsive pacing. Theaccelerometer amplifier output terminal is connected to the processor112. Because the leadless cardiac pacemaker 102 is attached to cardiacmuscle 104, the accelerometer 154 measures some acceleration due toheartbeats in addition to the desired activity signal. Processor 112performs sampling of the accelerometer output signal synchronously withthe cardiac cycle as determined by the cardiac sensing amplifier 132 andthe pulse generator 116. Processor 112 then compares accelerationsignals taken at the same relative time in multiple cardiac cycles todistinguish the part of the acceleration signal that results fromactivity and is not due to heart wall motion.

In other embodiments, the accelerometer 154 and accelerometer amplifier156 shown in FIG. 1B can be replaced with a temperature transducer suchas a thermistor and a signal conditioning amplifier connected toprocessor 112. In another embodiment, a pressure transducer and signalconditioning amplifier can be connected to processor 112. Temperature isnot sensitive to the cardiac cycle so that in such an activity sensorrate-responsive cardiac pacemaker embodiments, synchronous sampling withthe cardiac cycle is superfluous. Although pressure varies in thecardiac cycle, easily measured features of the pressure wave, forexample peak amplitude, peak-to-peak amplitude, peak rate of change(delta), and the like, can indicate the level of activity.

Also shown in FIG. 2, a cylindrical hermetic housing 110 is shown withannular electrodes 108 at housing extremities. In the illustrativeembodiment, the housing 110 can be composed of alumina ceramic whichprovides insulation between the electrodes. The electrodes 108 aredeposited on the ceramic, and are platinum or platinum-iridium.

Several techniques and structures can be used for attaching the housing110 to the interior or exterior wall of cardiac muscle 104.

A helix 226 and slot 228 enable insertion of the device endocardially orepicardially through a guiding catheter. A screwdriver stylet can beused to rotate the housing 110 and force the helix 226 into muscle 104,thus affixing the electrode 108A in contact with stimulable tissue.Electrode 108B serves as an indifferent electrode for sensing andpacing. The helix 226 may be coated for electrical insulation, and asteroid-eluting matrix may be included near the helix to minimizefibrotic reaction, as is known in conventional pacing electrode-leads.

In other configurations, suture holes 224 and 225 can be used to affixthe device directly to cardiac muscle with ligatures, during procedureswhere the exterior surface of the heart can be accessed.

The leadless cardiac pacemaker or pacemakers 102 can be configured todetect a natural cardiac depolarization, time a selected delay interval,and deliver an information-encoded pulse during a refractory periodfollowing the natural cardiac depolarization. By encoding information ina pacing pulse, power consumed for transmitting information is notsignificantly greater than the power used for pacing. Information can betransmitted through the communication channel with no separate antennaor telemetry coil. Communication bandwidth is low with only a smallnumber of bits encoded on each pulse.

In some embodiments, information can be encoded using a technique ofgating the pacing pulse for very short periods of time at specificpoints in the pacing pulse. During the gated sections of the pulse, nocurrent flows through the electrodes of a leadless cardiac pacemaker.Timing of the gated sections can be used to encode information. Thespecific length of a gated segment depends on the programmer's abilityto detect the gated section. A certain amount of smoothing or low-passfiltering of the signal can be expected from capacitance inherent in theelectrode/skin interface of the programmer as well as theelectrode/tissue interface of the leadless cardiac pacemaker. A gatedsegment is set sufficiently long in duration to enable accuratedetection by the programmer, limiting the amount of information that canbe transmitted during a single pacing pulse. Accordingly, a techniquefor communication can comprise generating stimulation pulses onstimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses. Encoding information ontothe pulses can comprise gating the stimulation pulses for selecteddurations at selected timed sections in the stimulation pulses wherebygating removes current flow through the stimulating electrodes andtiming of the gated sections encodes the information.

Another method of encoding information on pacing pulses involves varyingthe timing between consecutive pacing pulses in a pulse sequence. Pacingpulses, unless inhibited or triggered, occur at predetermined intervals.The interval between any two pulses can be varied slightly to impartinformation on the pulse series. The amount of information, in bits, isdetermined by the time resolution of the pulse shift. The steps of pulseshifting are generally on the order of microseconds. Shifting pulses byup to several milliseconds does not have an effect on the pacing therapyand cannot be sensed by the patient, yet significant information can betransmitted by varying pulse intervals within the microsecond range. Themethod of encoding information in variation of pulses is less effectiveif many of the pulses are inhibited or triggered. Accordingly, atechnique for communication can comprise generating stimulation pulseson stimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses comprising selectivelyvarying timing between consecutive stimulation pulses.

Alternatively or in addition to encoding information in gated sectionsand/or pulse interval, overall pacing pulse width can be used to encodeinformation.

The three described methods of encoding information on pacing pulses canuse the programmer to distinguish pacing pulses from the patient'snormal electrocardiogram, for example by recognition of the specificmorphology of the pacing pulse compared to the R-wave generated duringthe cardiac cycle. For example, the external programmer can be adaptedto distinguish a generated cardiac pacing pulse from a natural cardiacdepolarization in an electrocardiogram by performing comparative patternrecognition of a pacing pulse an an R-wave produced during a cardiaccycle.

Other attachment structures used with conventional cardiacelectrode-leads including tines or barbs for grasping trabeculae in theinterior of the ventricle, atrium, or coronary sinus may also be used inconjunction with or instead of the illustrative attachment structures.

Referring to FIG. 3, a pictorial view shows another embodiment of apulse generator that includes a cylindrical metal housing 310 with anannular electrode 108A and a second electrode 108B. Housing 310 can beconstructed from titanium or stainless steel. Electrode 108A can beconstructed using a platinum or platinum-iridium wire and a ceramic orglass feed-thru to provide electrical isolation from the metal housing.The housing can be coated with a biocompatible polymer such as medicalgrade silicone or polyurethane except for the region outlined byelectrode 108B. The distance between electrodes 108A and 108B should beselected to optimize sensing amplitudes and pacing thresholds. A helix226 and slot 228 can be used for insertion of the device endocardiallyor epicardially through a guiding catheter. In addition, suture sleeves302 and 303 made from silicone can be used to affix to the devicedirectly to cardiac muscle with ligatures.

In accordance with another embodiment of a pacing system 100, apacemaker configured as a rate-responsive leadless cardiac pacemaker 102comprising a housing 110, and multiple electrodes 108 coupled to thehousing 110. A pulse generator 116 hermetically contained within thehousing 110 and electrically coupled to the electrodes 108 and isconfigured for generating and delivering electrical pulses to theelectrodes 108. An activity sensor 154 is hermetically contained withinthe housing 110 and adapted to sense activity. A processor 112 ishermetically contained within the housing and communicatively coupled tothe pulse generator 116, the activity sensor 154, and the electrodes108. The processor 112 controls electrical pulse delivery at leastpartly based on the sensed activity and communicates with one or moredevices 106 external to the pacemaker 102 via signals conducted throughthe electrodes 108.

In various embodiments, the processor 112 and pulse delivery system 152transmits and/or receives information such as programmable parametersettings, event counts, power-supply voltage, power-supply current,rate-response control parameters adapted for converting an activitysensor signal to a rate-responsive pacing parameter.

Referring to FIG. 4, a typical output-pulse waveform for a conventionalpacemaker is shown. The approximately-exponential decay is due todischarge of a capacitor in the pacemaker through theapproximately-resistive load presented by the electrodes/tissueinterface and leads. Typically the generator output is capacitor-coupledto one electrode to ensure net charge balance. The pulse duration isshown as T0 and is typically 500 microseconds.

When the pacemaker 102 is supplying a pacing pulse but is not sendingdata for communication, the waveform can resemble that shown in FIG. 4.

In some embodiments, configurations, or conditions, the processor 112and pulse delivery system 152 are configured to generate and deliverelectrical energy with the stimulation pulse interrupted by at least onenotch that conveys information to a device 106 external to the pacemaker102.

Referring to FIG. 5, a time waveform graph depicts an embodiment of asample output-pacing pulse waveform adapted for communication. Theoutput-pulse waveform of the illustrative leadless pacemaker 102 isshown during a time when the pacemaker 102 is sending data forcommunication and also delivering a pacing pulse, using the same pulsegenerator 116 and electrodes 108 for both functions.

FIG. 5 shows that the pulse generator 102 has divided the output pulseinto shorter pulses 501, 502, 503, 504; separated by notches 505, 506,and 507. The pulse generator 102 times the notches 505, 506, and 507 tofall in timing windows W1, W2, and W4 designated 508, 509, and 511respectively. Note that the pacemaker 102 does not form a notch intiming window W3 designated 510. The timing windows are each shownseparated by a time T1, approximately 100 microseconds in the example.

As controlled by processor 112, pulse generator 116 selectivelygenerates or does not generate a notch in each timing window 508, 509,510, and 511 so that the device 102 encodes four bits of information inthe pacing pulse. A similar scheme with more timing windows can sendmore or fewer bits per pacing pulse. The width of the notches is small,for example approximately 15 microseconds, so that the delivered chargeand overall pulse width, specifically the sum of the widths of theshorter pulses, in the pacing pulse is substantially unchanged from thatshown in FIG. 4. Accordingly, the pulse shown in FIG. 5 can haveapproximately the same pacing effectiveness as that shown in FIG. 4,according to the law of Lapique which is well known in the art ofelectrical stimulation.

In a leadless cardiac pacemaker, a technique can be used to conservepower when detecting information carried on pacing pulses from otherimplanted devices. The leadless cardiac pacemaker can have a receivingamplifier that implements multiple gain settings and uses a low-gainsetting for normal operation. The low-gain setting could beinsufficiently sensitive to decode gated information on a pacing pulseaccurately but could detect whether the pacing pulse is present. If anedge of a pacing pulse is detected during low-gain operation, theamplifier can be switched quickly to the high-gain setting, enabling thedetailed encoded data to be detected and decoded accurately. Once thepacing pulse has ended, the receiving amplifier can be set back to thelow-gain setting. For usage in the decoding operation, the receivingamplifier is configured to shift to the more accurate high-gain settingquickly when activated. Encoded data can be placed at the end of thepacing pulse to allow a maximum amount of time to invoke the high-gainsetting.

In some embodiments, configurations, or conditions, the processor 112and pulse delivery system 152 are configured to generate and deliverelectrical energy with the stimulation pulse that conveys information toa device 106 external to the pacemaker 102 in designated codes encodingthe information in modulation of off-time between pacing pulses.

As an alternative or in addition to using notches in the stimulationpulse, the pulses can be generated with varying off-times, specificallytimes between pulses during which no stimulation occurs. The variationof off-times can be small, for example less than 10 milliseconds total,and can impart information based on the difference between a specificpulse's off-time and a preprogrammed off-time based on desired heartrate. For example, the device can impart four bits of information witheach pulse by defining 16 off-times centered around the preprogrammedoff-time. FIG. 6 is a graph showing a sample pulse generator outputwhich incorporates a varying off-time scheme. In the figure, time T_(P)represents the preprogrammed pulse timing. Time T_(d) is the delta timeassociated with a single bit resolution for the data sent by the pulsegenerator. The number of T_(d) time increments before or after themoment specified by T_(P) gives the specific data element transmitted.The receiver of the pulse generator's communication has advanceinformation of the time T_(P). The communication scheme is primarilyapplicable to overdrive pacing in which time T_(P) is not changing basedon detected beats.

In some embodiments, configurations, or conditions, the processor 112and pulse delivery system 152 are configured to generate and deliverelectrical energy with the stimulation pulse that conveys information toa device 106 external to the pacemaker 102 in designated codes encodingthe information in pacing pulse width.

FIG. 5 depicts a technique in which information is encoded in notches inthe pacing pulse. FIG. 6 shows a technique of conveying information bymodulating the off-time between pacing pulses. Alternatively or inaddition to the two illustrative coding schemes, overall pacing pulsewidth can be used to impart information. For example, a paced atrialbeat may exhibit a pulse width of 500 microseconds and an intrinsicatrial contraction can be identified by reducing the pulse width by 30microseconds. Information can be encoded by the absolute pacing pulsewidth or relative shift in pulse width. Variations in pacing pulse widthcan be relatively small and have no impact on pacing effectiveness.

To ensure the leadless cardiac pacemaker functions correctly, a specificminimum internal supply voltage is maintained. When pacing tankcapacitor charging occurs, the supply voltage can drop from apre-charging level which can become more significant when the batterynears an end-of-life condition and has reduced current sourcingcapability. Therefore, a leadless cardiac pacemaker can be constructedwith a capability to stop charging the pacing tank capacitor when thesupply voltage drops below a specified level. When charging ceases, thesupply voltage returns to the value prior to the beginning of tankcapacitor charging.

In another technique, the charge current can be lowered to prevent thesupply voltage from dropping below the specified level. However,lowering the charge current can create difficulty in ensuring pacingrate or pacing pulse amplitude are maintained, since the lower chargecurrent can extend the time for the pacing tank capacitor to reach atarget voltage level.

The illustrative scheme for transmitting data does not significantlyincrease the current consumption of the pacemaker. For example, thepacemaker could transmit data continuously in a loop, with noconsumption penalty.

The illustrative example avoids usage of radiofrequency (RF)communication to send pacing instructions to remote electrodes on abeat-to-beat basis to cause the remote electrodes to emit a pacingpulse. RF communication involves use of an antenna andmodulation/demodulation unit in the remote electrode, which increaseimplant size significantly. Also, communication of pacing instructionson a beat-to-beat basis increases power requirements for the main bodyand the remote electrode. In contrast, the illustrative system andstimulator do not require beat-to-beat communication with anycontrolling main body.

The illustrative leadless pacemaker 102 includes an internal powersource that can supply all energy for operations and pulse generation.In contrast, some conventional implanted pulse generators have remotepacing electrodes that receive some or all energy from an energy sourcethrough an RF induction technique, an energy transfer scheme thatemploys a large loop antenna on the remote electrode which increasessize significantly. In addition, energy transfer with the RF inductiontechnique is inefficient and is associated with a significant increasein battery size of the energy source. In contrast, the illustrativeleadless pacemaker 102 uses an internal battery and does not requireenergy to be drawn from outside sources. Also in the conventionalsystem, the energy source receives sensing information by RFcommunication from the remote electrodes and sends pacing instructionsto the electrodes on a beat-to-beat basis in a configuration that usesan addressing scheme in which the identity of specific remote pacingelectrodes is stored in the energy source memory. The conventionalmethod can also be inefficient due to overhead for transmitting anidentification number from/to a generic pacing electrode at implantand/or during sensing. The illustrative leadless pacemaker 102 avoidssuch overhead through a structure in which pulse generationfunctionality is independent within a single implantable body.

Another conventional technology uses a system of addressable remoteelectrodes that stimulate body tissue without requiring a main body tosend commands for individual stimulations. The remote electrodes arespecified to be of a size and shape suitable for injection rather thanfor endocardial implantation. A controller sets operating parameters andsends the parameters to remote electrodes by addressable communication,enabling the remote electrodes function relatively autonomously whileincurring some overhead to controller operations. However, the remoteelectrodes do not sense or monitor cardiac information and rely on themain body to provide sensing functionality. In contrast, theillustrative leadless pacemaker 102 combines pacing and sensing ofintrinsic cardiac activity in a single implantable body.

Referring again to FIG. 1B, the circuit 132 for receiving communicationvia electrodes 108 receives the triggering information as described andcan also optionally receive other communication information, either fromthe other implanted pulse generator 106 or from a programmer outside thebody. This other communication could be coded with a pulse-positionscheme as described in FIG. 5 or could otherwise be a pulse-modulated orfrequency-modulated carrier signal, preferably from 10 kHz to 100 kHz.

With regard to operating power requirements in the leadless cardiacpacemaker 102, for purposes of analysis, a pacing pulse of 5 volts and 5milliamps amplitude with duration of 500 microseconds and a period of500 milliseconds has a power requirement of 25 microwatts.

In an example embodiment of the leadless pacemaker 102, the processor112 typically includes a timer with a slow clock that times a period ofapproximately 10 milliseconds and an instruction-execution clock thattimes a period of approximately 1 microsecond. The processor 112typically operates the instruction-execution clock only briefly inresponse to events originating with the timer, communication amplifier134, or cardiac sensing amplifier 132. At other times, only the slowclock and timer operate so that the power requirement of the processor112 is no more than 5 microwatts.

For a pacemaker that operates with the aforementioned slow clock, theinstantaneous power consumption specification, even for acommercially-available micropower microprocessor, would exceed thebattery's power capabilities and would require an additional filtercapacitor across the battery to prevent a drop of battery voltage belowthe voltage necessary to operate the circuit. The filter capacitor wouldadd avoidable cost, volume, and potentially lower reliability.

For example, a microprocessor consuming only 100 microamps would requirea filter capacitor of 5 microfarads to maintain a voltage drop of lessthan 0.1 volt, even if the processor operates for only 5 milliseconds.To avoid the necessity for such a filter capacitor, an illustrativeembodiment of a processor can operate from a lower frequency clock toavoid the high instantaneous power consumption, or the processor can beimplemented using dedicated hardware state machines to supply a lowerinstantaneous peak power specification.

In a pacemaker, the cardiac sensing amplifier typically operates with nomore than 5 microwatts.

An accelerometer amplifier, or other general purpose signal conditioningamplifier, operates with approximately 10 microwatts.

A communication amplifier at 100 kHz operates with no more than 25microwatts. The battery ammeter and battery voltmeter operate with nomore than 1 microwatt each.

A pulse generator typically includes an independent rate limiter with apower consumption of no more than 2 microwatts.

The total power consumption of the pacemaker is thus 74 microwatts, lessthan the disclosed 75-microwatt battery output.

Improvement attained by the illustrative cardiac pacing system 100 andleadless cardiac pacemaker 102 is apparent.

The illustrative cardiac pacing system 100 enables encoding optionaloutgoing communication in the pacing pulse, so that the outgoingcommunication power requirement does not exceed the pacing currentrequirement, approximately 25 microwatts.

The illustrative leadless cardiac pacemaker 102 can have sensing andprocessing circuitry that consumes no more than 10 microwatts as inconventional pacemakers.

The described leadless cardiac pacemaker 102 can have an incomingcommunication amplifier for receiving triggering signals and optionallyother communication which consumes no more than 25 microwatts.

Furthermore, the leadless cardiac pacemaker 102 can have a primarybattery that exhibits an energy density of at least 3 watt-hours percubic centimeter (W·h/cc).

Referring to FIG. 7, a schematic flow chart depicts an embodiment of amethod 700 for operating an activity sensor in a rate-responsive cardiacpacemaker. A leadless cardiac pacemaker that includes a rate-responsesensor is implanted 702 in contact with cardiac muscle. An activitysignal is measured 704 using the rate-response sensor. The activitysignal includes an artifact signal that results from cardiac musclemotion. The activity signal is sampled 706 synchronously with a cardiaccycle. The activity signal is monitored 708 at identified points in thecardiac cycle. The artifact signal is removed 710 from the activitysignal based on the monitoring. In various embodiments, the activitysignal can be measured using an accelerator, thermistor, or pressuresensor.

Referring to FIG. 8, a schematic flow chart depicts an embodiment of amethod 800 for setting operating parameters in a rate-responsive cardiacpacemaker. The method 800 comprises sensing 802 electrical signalsconducted through a patient's body, decoding 804 information encoded inthe electrical signals, and storing the result. An activity signal issensed 806 within the pacemaker. The activity sensor signal is converted808 to a rate-responsive pacing parameter as a function of the storedinformation encoded in the electrical signals. Pacing pulse delivery iscontrolled 810 as a function of the rate-responsive pacing parameter.

In some embodiments, information is encoded, for example, as a binarycode in one or more notches interrupting a stimulation pulse.Information can otherwise or also be encoded in selected or designatedcodes as variations in pacing pulse width of a stimulation pulse.Information can also be conveyed as electrical energy in a stimulationpulse in designated codes encoding the information in modulation ofoff-time between pacing pulses.

Terms “substantially”, “essentially”, or “approximately”, that may beused herein, relate to an industry-accepted tolerance to thecorresponding term. Such an industry-accepted tolerance ranges from lessthan one percent to twenty percent and corresponds to, but is notlimited to, component values, integrated circuit process variations,temperature variations, rise and fall times, and/or thermal noise. Theterm “coupled”, as may be used herein, includes direct coupling andindirect coupling via another component, element, circuit, or modulewhere, for indirect coupling, the intervening component, element,circuit, or module does not modify the information of a signal but mayadjust its current level, voltage level, and/or power level. Inferredcoupling, for example where one element is coupled to another element byinference, includes direct and indirect coupling between two elements inthe same manner as “coupled”.

While the present disclosure describes various embodiments, theseembodiments are to be understood as illustrative and do not limit theclaim scope. Many variations, modifications, additions and improvementsof the described embodiments are possible. For example, those havingordinary skill in the art will readily implement the steps necessary toprovide the structures and methods disclosed herein, and will understandthat the process parameters, materials, and dimensions are given by wayof example only. The parameters, materials, and dimensions can be variedto achieve the desired structure as well as modifications, which arewithin the scope of the claims. Variations and modifications of theembodiments disclosed herein may also be made while remaining within thescope of the following claims. For example, although the description hassome focus on CRT, the pacemaker, system, structures, and techniques canotherwise be applicable to other uses, for example multi-site pacing forprevention of tachycardias in the atria or ventricles. Phraseology andterminology employed herein are for the purpose of the description andshould not be regarded as limiting. With respect to the description,optimum dimensional relationships for the component parts are to includevariations in size, materials, shape, form, function and manner ofoperation, assembly and use that are deemed readily apparent and obviousto one of ordinary skill in the art and all equivalent relationships tothose illustrated in the drawings and described in the specification areintended to be encompassed by the present description. Therefore, theforegoing is considered as illustrative only of the principles ofstructure and operation. Numerous modifications and changes will readilyoccur to those of ordinary skill in the art whereby the scope is notlimited to the exact construction and operation shown and described, andaccordingly, all suitable modifications and equivalents may be included.

1. A method of pacing a heart, comprising: sensing a parameter with asensor of a leadless pacemaker implanted in the heart; generatingelectrical pulses with a pulse delivery system of the leadlesspacemaker; delivering the electrical pulses through electrodes of theleadless pacemaker to stimulate the heart at least partly based on thesensed parameter; and communicating encoded information signals to adevice external to the leadless pacemaker through the electrodes of theleadless pacemaker.
 2. The method of claim 1 wherein the delivering stepfurther comprises delivering the electrical pulses to the heart throughelectrodes of the leadless pacemaker at least partly based on the sensedparameter and partly based on communication signals received via theelectrodes.
 3. The method of claim 1 wherein the sensing step furthercomprises sensing temperature with the sensor of the leadless pacemaker.4. The method of claim 1 wherein the sensing step further comprisessensing pressure with the sensor of the leadless pacemaker.
 5. Themethod of claim 1 wherein the information signals are selected from agroup consisting of programmable parameter settings, event counts,power-supply voltage, power-supply current, and rate-response controlparameters adapted for converting a sensor signal to a rate-responsivepacing parameter.
 6. The method of claim 1 wherein the communicatingstep further comprises communicating encoded information signals to thedevice in designated codes encoding the information in pacing pulsewidth.
 7. The method of claim 1 wherein the communicating step furthercomprises communicating encoded information signals to the device indesignated codes encoding the information in modulation of off-timebetween pacing pulses.
 8. The method of claim 1 further comprising:invoking a low-gain setting in a receiving amplifier/filter of theleadless pacemaker for normal operation and detecting presence of atleast one pulse; and invoking a high-gain setting for detecting anddecoding information encoded in the detected at least one pulse.
 9. Amethod of operating pacemaker system, comprising: sensing with aleadless pacemaker encoded electrical signals conducted through cardiactissue; decoding and storing information in the encoded electricalsignals; sensing a patient sensor signal within the pacemaker;converting the patient sensor signal to a rate-responsive pacingparameter as a function of the stored information; and controllingpacing pulse delivery as a function of the rate-responsive pacingparameter.
 10. The method of claim 9 further comprising: generating astimulation pulse in the leadless pacemaker; delivering electricalenergy through electrodes of the leadless pacemaker to the cardiactissue with the stimulation pulse to stimulate cardiac tissue; andcommunicating encoded information through the electrodes of the leadlesspacemaker to a device external to the leadless pacemaker.
 11. The methodof claim 9 further comprising: generating a stimulation pulse in theleadless pacemaker; delivering electrical energy through electrodes ofthe leadless pacemaker to the cardiac tissue with the stimulation pulseto stimulate cardiac tissue; and encoding information in a pacing pulsewidth of the stimulation pulse to communicate with a device external tothe leadless pacemaker.
 12. The method of claim 9 further comprising:generating a stimulation pulse in the leadless pacemaker; deliveringelectrical energy through electrodes of the leadless pacemaker to thecardiac tissue with the stimulation pulse to stimulate cardiac tissue;and encoding information in modulation of off-time between stimulationpulses to communicate with a device external to the leadless pacemaker.13. A method for operating a pacemaker comprising: implanting a leadlesscardiac pacemaker into cardiac tissue; measuring a patient signal with apatient sensor, the patient signal including an artifact signalresulting from cardiac muscle motion; sampling the patient signalsynchronously with a cardiac cycle; monitoring the patient signal atidentified points in the cardiac cycle; and removing the artifact signalfrom the patient signal based on the monitoring.
 14. The methodaccording to claim 13 further comprising measuring the patient signalusing a temperature sensor.
 15. The method according to claim 13 furthercomprising measuring the patient signal using an accelerometer.
 16. Themethod according to claim 13 further comprising measuring the patientsignal using a pressure sensor.